According to known practice a fuse is provided which is capable of interrupting all currents from the rated maximum interrupting rating down to the rated minimum interrupting rating and which is connected in series with a so-called weak link expulson fuse which is specially designed to effect interruption of currents below the value of the minimum interrupting current rating of the current limiting fuse. Obviously it is desirable to eliminate the practice of requiring the use of two fuses.
Another widely used system for maintaining low temperature operation of a fuse utilizing silver fusible elements utilizes the so-called Metcalf or M effect. In this type of fuse, a silver ribbon is modified by the placement of a small deposit of tin or tin alloy at one point on the silver ribbon to form an eutectic alloy with the silver to promote melting at that point on the ribbon when it reaches a temperature of approximately 230.degree. C. In the absence of the M effect, silver elements melt at a temperature of approximately 960.degree. C. Obviously melting temperatures of such a high order of magnitude without the eutectic effect are destructive to the fuse and are counter productive to desirable fuse operation. Where the M effect is utilized, the melting of the silver ribbon is localized at that point and the resulting arc and continued current flow must increase the ribbon temperature by an additional 700.degree. C. approximately. In addition nonmelting current flows can cause the alloy formation at the M spot to produce a permanent change in the fuse melting characteristic.
In one modification of the eutectic design, a parallel slave element is provided for the purpose of initiating two further breaks in the fusible element following the initial establishment of melting at the M spot. Such structure limits the points of melting to three and obviously is not altogether desirable and also introduces a degree of complication.
In accordance with another practice, a core is provided on which the fusible elements are wound and is constructed of gas evolving material. Where this type of structure is used venting of the housing is required. If the housing is vented of course the interrupting operation is not isolated and can result in failure of the fuse or damage to other apparatus.
Still another type of fuse utilizes a silver element connected in series with a tin element. The tin element is enclosed in an insulating tube and is expelled from the tube into the filler element to achieve low current interruption. Obviously this structure involves a measure of complication, and in addition is only suited for lower current ratings.
Still another practice has involved thermally insulating a silver wire section arranged in series with a silver ribbon. The heat concentration promotes earlier melting of the silver wire. It adds substantially to the cost of the fuse.
Still another practice has involved the use of a gold alloy in an arc quenching tube connected in series with a silver element so as to aid in the interruption of low currents.
From the above discussion of prior practices, it is evident that there are difficulties involved in interrupting low values of current. Furthermore the requirement for interrupting low currents has added substantially to the complexity of fuse designs, to their size and cost. It also limits their maximum current ratings and their application.
A fuse having fusible elements formed of cadmium is free of most of the objectionable features of the prior art but is not entirely satisfactory because of the tendency of cadmium to sublimate.
U.S. Pat. No. 3,838,376 Norholm discloses a fuse in which a core of cadmium is embedded within and partially surrounded by aluminum. The function of this structure is to explode and thus to interrupt an electric current and the sheath is thick and heavy.